A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A conventional lithographic apparatus may comprise a collector to collect radiation from a radiation source. The collector may be arranged to transmit the collected radiation to an illumination system. Such an illumination system may be arranged to provide a projection beam of radiation, which is used to apply a desired pattern onto a target portion of a substrate. The collector may comprise, for example, suitable radiation reflective mirror shells. Particularly, the collector may be arranged to focus incoming radiation, received from the radiation source, onto a small focusing area or focus point.
For example, from United States patent application publication no. US 2005/0157284, an apparatus comprises a heater to heat a collector when the collector receives substantially no radiation from the radiation source. Further, U.S. Pat. No. 6,859,259 describes an apparatus wherein a reflector creates a low intensity zone behind an aperture.
A heat load, resulting from incoming radiation, may lead to thermal gradients of the collector. These temperature gradients may cause collector deformation, which may lead to a loss of optical performance.